The bypass mode of operating a steam turbine, although advantageous in many respect, presents a unique set of problems as this mode of operation is extended to those turbines at the larger end of the size spectrum. In particular, at low-load (including no-load) operating conditions, the larger machines encounter a problem known variously as "windage loss heating" or as "rotation loss heating". This kind of heating is most likely to occur at the exhaust end of the high pressure section of the turbine as a result of the combination of high back-pressure at that point from the bypass steam flow and the relatively low flow of steam through the turbine's high pressure section. Windage loss heating, if uncontrolled, may increase the steam temperature to excessive levels and is, therefore, potentially damaging to the turbine.
In U.S. patent application Ser. No. 105,019, now U.S. Pat. No. 4,309,873, which is assigned to the assignee of the instant application, K. W. Koran and W. T. Parry disclose and claim a method and apparatus for preventing rotation loss heating. According to their method, a reverse flow of steam is provided through the high pressure section of the turbine so that windage losses are taken away by cooling steam flow. The reverse flow of steam is preferably taken from the high pressure bypass line just ahead of the steam reheater and is then passed backwards through the turbine's high pressure (HP) section from the last stages through the first stage. The reverse steam flow path includes a reverse flow valve to admit the cooling steam to the HP section and a ventilator valve connected to discharge the cooling steam from the HP section. The control valves through which steam is admitted to the turbine in the conventional, forward flow direction must, of course, be closed when the reverse flow path is used.
During the reverse flow regime, admission of steam to the lower pressure sections of the turbine provide the forward driving energy to sustain whatever load may be applied. By proper balance of steam flow, the reverse flow technique is effective to eliminate excessive windage loss heating in the lower pressure (LP) sections of a turbine as well as in the high pressure section.
At some point during turbine operation, when load on the turbine has been increased to the point at which steam flow in the forward direction of the high pressure section can be established without excessive temperatures in either the HP or LP sections, the ventilator valve is closed and the admission control valves are opened.
The transfer from one steam flow regime to the other requires knowledge of turbine operating parameters and an exercise of judgement, based on the operating parameters, as to when the transfer in steam flow direction can most effectively be made. A transfer which is either late or premature is undesirable from considerations of both operating efficiency and minimization of chances of overheating damage to the machine.
Thus, it is a principal objective of the present invention to provide a control system for ascertaining certain preselected operating conditions of the turbine and for automatically transferring to either a forward or a reverse steam flow through the high pressure section thereof depending on the status of such conditions.